A DECADE ago trainer Derek Cheng Tai-chee had won the Hongkong Derby with a horse called Football, ridden by good friend Joe Mercer, and son Paul was champion apprentice. They were vying with the Moores to be hailed the ''first family'' of Hongkong racing. The same Derek was champion jockey of Hongkong himself 20 years ago, when he wound up a riding career that had seen him reign supreme. But last week what might have been an enduring Hongkong racing dynasty ended when Paul ''resigned'' as assistant trainer to Wong Tang-ping. Derek's career as a trainer ended ingloriously in 1986 when he was disqualified for four years on betting offences, turned up during the Shanghai Syndicate investigation. He later applied to be licensed in Hongkong and then Macau as a trainer, but without success. Paul quit the saddle to become an assistant trainer but with little success. Time changes everything. THE Sha Tin weighing room was buzzing yesterday with the news of Brent Thomson's decision to quit. But the man in the middle of it all was taking it in his stride - as he usually does. Thomson has seen the ups and downs of racing but he faces the world with a ready smile and Hongkong racing will be the poorer for his going. ''Life goes on,'' said Thomson, adding: ''Just think, I won't have trackwork to interfere with my life in Hongkong for the next three weeks or so. That has to be a bonus.'' THERE'S an old crack that goes ''You can always tell an Aussie - but you can't tell him very much''. Not quite the case, it would seem, with two of our more talented riders from the Antipodes. John Marshall, seemingly unstoppable, is a left-handed whip rider of some note. His whirlwind action had punters ''wah-ing'' in his first months in the territory. But now the former Sydney champion is using it readily in the right hand. ''Yes, I have had a chance to practise a bit more up here. I'm not saying I'm as good with it in the right as in the left, but I'm getting there.'' And Darren Gauci was never noted for using his left hand to punch a horse out hands and heels - but he is doing it now. Nice to know that they have been suitably enriched - professionally - from their Hongkong experiences. TIMES must be tough in Manawatu. The prominent New Zealand racing club has been officially linked with the Jockey Club since 1980 and their Trophy race is a regular fixture on the calendar. It is normally presented by a leading official of the New Zealand club, but none was on hand yesterday to do the honours. Deputising was Matthew Oram, president of the Hongkong Racehorse Owners Association, whose family has links with New Zealand.